1. Field of the Invention
This invention relates to isocyanate compositions and is more particularly concerned with emulsifiable polyisocyanates and with processes for their preparation and their use as binder for the preparation of particle board, hard board, foam scrap, wafer board and foundry cores.
2. Description of the Prior Art
The use of organic polyisocyanates, particularly polymethylene polyphenyl polyisocyanates, as binders or components of binders for the preparation of particle boards and like products is now widely recognized; see, for example, U.S. Pat. Nos. 3,428,592; 3,440,189; 3,557,263; 3,636,199; 3,870,665; 3,919,017; 3,930,110; and 4,257,995. In a particular mode of application the polyisocyanates have been used in the form of aqueous emulsions. The latter are applied, e.g. in the form of a spray, to the particles of wood or like cellulosic material which are employed to form the composite. The coated particles are then molded by subjecting them to the action of heat and pressure in order to prepare the final composite. In order to use the polyisocyanates in the form of aqueous emulsions in the above manner it is desirable that the emulsion be stable over a period of at least several hours and, preferably, several days. Thus, in commercial production of particle boards, there can be unscheduled interruptions in operations for a variety of reasons. If the aqueous emulsion utilized as the binder has only very limited stability, it is possible that a prolonged and unexpected interruption in the composite manufacturing process can result in deterioration of the binder emulsion to the point at which the latter has to be discarded in its entirety. The use of the term "stable" in the above context encompasses not only the maintenance of the emulsified state but the reaction, or rather lack of reaction, of the polyisocyanate with the water used in making the emulsion.
A number of aqueous emulsions of polyisocyanates have been described whose primary intended use is as binders for particle board. Illustratively, German Specification 2703271 describes the use of polyglycols having molecular weights from 4000 to 20,000 as emulsifying agents for polyisocyanates of which methylenebis(phenyl isocyanate) is specifically exemplified. U.S. Pat. No. 4,260,532 reports that the stability of the emulsions in the German specification can be improved by replacing a part or the whole of the polyglycol with a water-soluble polyamide such as polyvinylpyrrolidone.
U.S. Pat. No. 3,996,154 relates to the preparation of aqueous emulsions of polyisocyanates in which the emulsifying agent is derived by reaction of a minor amount of the polyisocyanate with the mono-lower-alkyl ether of a polyethylene glycol. The emulsifying agent can be preformed or formed in situ by adding the polyethylene glycol ether to the bulk of the polyisocyanate. The polyisocyanate may be present as prepolymer obtained by reacting the polyisocyanate with a polyol. The emulsions prepared as described in the above reference are shown in U.S. Pat. No. 4,110,397 to be useful as binders in the preparation of particle board.
German Offenlegungsschrift 2,921,681 (and corresponding European application 19844) describes aqueous emulsions of polyisocyanate in which the emulsifying agent is the product of reaction of a polyisocyanate and a mono-alcohol which comprises a polyethylene glycol having one of its hydroxyl groups blocked or replaced by a variety of different groups none of which, with the sole exception of an epoxy group, is reactive with isocyanate. The companion German Offenlegungsschrift 2,921,726 (and corresponding European application 19860) describes the use of these emulsions as particle board binders.
We have now found that emulsifiable isocyanate compositions with advantageous properties can be prepared by reaction of polymethylene polyphenyl polyisocyanates with a particular group of derivatives of block copolymers of ethylene oxide and propylene oxide and that aqueous emulsions having advantageous properties can be obtained by dispersing said isocyanates in water.